High voltage and high efficiency polymer electrolytic capacitors

ABSTRACT

A capacitor, and method of making a capacitor, is provided wherein the capacitor has exceptionally high break down voltage. The capacitor has a tantalum anode with an anode wire attached there to. A dielectric film is on the tantalum anode. A conductive polymer is on the dielectric film. An anode lead is in electrical contact with the anode wire. A cathode lead is in electrical contact with the conductive polymer and the capacitor has a break down voltage of at least 60 V.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of pending U.S. patentapplication Ser. No. 12/426,516 filed Apr. 20, 2009 which isincorporated herein by reference.

BACKGROUND

The present invention is related to an electrolytic capacitor. Morespecifically the present invention is related to an electrolyticcapacitor comprising intrinsically conductive polymeric cathode layerscapable of achieving a high break down voltage (BDV) which were notpreviously available with polymeric cathode layers.

Solid electrolytic capacitors with intrinsically conductive polymers asthe cathode material have been widely used in the electronics industrydue to their advantageous low equivalent series resistance (ESR) and“non-burning/non-ignition” failure mode. Intrinsically conductivepolymer, more commonly known as conductive polymer, is electricallyconductive in the molecular level. In other words, a single molecule (apolymer chain) of this type of polymer is conductive, whichdistinguishes itself from other groups of polymeric materials whoseelectrical conductivity is imported from the presence of foreignconductive particles. The example of the latter is polyester(non-conductive) filled with carbon back (conductive particles). Theintrinsically conducting polymer can exist in many physical formsincluding solid, solution, and liquid dispersion.

The backbone of a conductive polymer consists of a conjugated bondingstructure. The polymer can exist in two general states, an undoped,non-conductive state, and a doped, conductive state. In the doped state,the polymer is conductive but of poor processibility due to a highdegree of conjugation along the polymer chain, while in its undopedform, the same polymer loses its conductivity but can be processed moreeasily because it is more soluble. When doped, the polymer incorporatesanionic moieties as constituents on its positively charged backbone. Inorder to achieve high conductivity, the conductive polymers used in thecapacitor must be in doped form after the completion of processing,although during the process, the polymer can be undoped/doped to achievecertain process advantages.

Various types of conductive polymers including polypyrrole, polyaniline,and polythiophene are described for use in Ta capacitors. The majordrawback of conductive polymer capacitors, regardless of the types ofconductive polymers employed, is their relatively low working voltagecompared to their MnO₂ counterparts. Since their introduction to themarket, the working voltages of Polymer Ta capacitors has been limitedto 25 V, while the working voltages of Solid Ta capacitors (MnO₂cathode) available on the market can reach 75 V and the working voltageof Wet Ta capacitors can reach 150V. This limitation has madeapplications of polymer Ta capacitors in high voltage circuitsimpossible which is where the combination of low ESR and non-burningfailure mode are most critical.

During manufacture the Ta powder is mechanically pressed to make Tametal pellets. The pellets are subsequently sintered at high temperatureunder vacuum. The sintered anodes are then anodized in a liquidelectrolyte at elevated temperature to form a cohesive dielectric layer(Ta₂O₅) on the anode surface. Increasing formation voltage increases thedielectric thickness, which determines the maximum voltage the anodescan withstand. Polymer cathodes are conventionally applied to tantalumcapacitors by synthesis from the monomer and an oxidizing agent. This isknown as ‘in-situ’ polymerization. Typically the anodes are prepared bythe steps of dipping in oxidizing agent, drying, dipping in monomer,reacting the monomer and oxidizing agent to form conductive polymer andwashing of byproducts not necessarily in this order. Optionally, areform step may be applied after washing to reduce DC leakage offinished capacitors.

With reference to FIG. 2, there is a large increase in leakage currentat about 35 V for both capacitors comprising a polymeric cathode(In-Situ Ctrl and In-Situ Test) despite the formation voltage being 125V. This can be compared to MnO₂ cathode, which does not show anappreciable increase of leakage current until about 70 V, and the wet,sulfuric acid, cathode which does not show an appreciable increase inleakage current until about 120 V. The dielectrics for both the MnO₂ andwet devices were also formed to 125 V. Thus, despite the high formationvoltage, it would be difficult to rate polymer capacitors above about 25V.

After formation of the polymer coating graphite and silver are appliedto allow adhesion to the cathode lead. The manufacturing process is thencontinued by assembling, molding and tested the capacitors.

The rating voltage for Ta capacitors, or the working voltage allowed forreliable operation, is primarily a function of dielectric thickness.Dielectric thickness is controlled by the formation voltage. Increasingthe formation voltage increases the dielectric thickness. It isestimated that for every volt applied during the dielectric formationprocess, about 1.7˜2 nm of dielectric is formed on the surface. For agiven anode, increasing dielectric thickness is at a cost of capacitanceloss since the anode capacitance is inversely proportional to dielectricthickness. It is a common practice for solid Ta capacitor manufacturersto use a formation voltage which is 2.5 to 4 times higher than the anoderated voltage. This ensures high reliability during applications. Forexample, a 10V rated capacitor often employs an anode formed at 30V.

A plot of the BDV versus the formation voltage for a wide range of Tacapacitors including both polymer (polyethyldioxythiophene, or PEDOT)and MnO₂ based capacitors is shown in FIG. 1.

As shown FIG. 1, in the low formation voltage region (<30V), the BDV ofboth polymer and MnO₂ capacitors are close to the anode formationvoltages. However, there is a trend of divergence in terms of BDVbetween MnO₂ and polymer capacitors as formation voltage increases fromabout 80V to 200V. In this range, while the BDV of MnO₂ parts stillincreases with increasing formation voltage, the BDV of polymercapacitor shows a mostly flat pattern. This has been interpreted in theart to indicate a limit of about 50V which is almost unaffected by theincreasing formation voltage. Increasing dielectric thickness, which isthe most important and commonly used approach to make high voltagecapacitors, is virtually ineffective for making high voltage polymercapacitors beyond about 25V ratings. Due to this phenomenon the Taindustry has had difficulty producing reliable conducting polymercapacitors for use above 25 V. A 35V rated capacitor, for example, wouldrequire a BDV of far greater than 50V to ensure its long termreliability (e.g. 35V rated MnO₂ parts have an average BDV of 95V. Whilenot limited to any theory it is postulated that the polymer/dielectricinterface can cause the differences in BDV.

In recent years conductive polymers have received considerableattention. This material is a suspension of conductive polymer in asolvent. Instead of the conventional method of applying the conductivepolymer by in-situ synthesis from the monomer and an oxidizing agent,the polymer can now be applied by dipping in the slurry and removing thesolvent. Again with reference to FIG. 2 the leakage current vs. voltagebehavior of the slurry polymer cathode compares favorably to the in-situformed cathode. A significant improvement is obtained. Large leakagecurrents do not flow until about 75 V. Thus, devices of 35 V ratings canbe manufactured. However, even with these polymer slurry cathodes,leakage current performance is still unsatisfactory. Wet tantalumdevices with formation voltages of 125 V would be expected to be ratedup to 70 V. Thus, a significant improvement in rated voltage of tantalumcapacitors with polymer cathodes is still needed if they are to competewith wet tantalum capacitors in higher voltage ratings.

It is known in the art, that reducing the oxygen and carbon content ofthe anodes leads to the formation of a better quality dielectric. It hasbeen demonstrated on tantalum capacitors with wet and MnO₂ cathodes thatreduction of oxygen and carbon contents can significantly improve thelong term reliability of the capacitors. However, little improvement isnoted in the initial performance. This is show in FIGS. 3 a and 3 b fora tantalum capacitor with a MnO₂ cathode. It would not be expected thatreduction of oxygen and carbon contents of the anode would improve theinitial performance of tantalum capacitors with polymer cathodes.Indeed, applying these state of the art processing techniques incombination with conventional methods of depositing conductive polymerleads to only a very small improvement in performance. FIG. 1 shows thatapplying these techniques leads to only a few volts improvement in wherethe leakage current increases as indicated in the ‘in-situ test’. Thecombination of polymer deposited by an in-situ method and the best anodetechnology still leads to a device that cannot compete in rated voltagewith MnO₂ or wet capacitors. Thus, these capacitors still fall short ofmeeting the goal of replacing MnO₂ or wet capacitors with a lower ESRdevice at rated voltages above 25 V.

There has been a long standing desire in the art to provide a capacitorcomprising a conducting polymeric cathode suitable for use at higherrated voltages. Through diligent research the present inventors haveachieved what was previously not considered feasible.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a capacitor comprising aconducting polymer with a high breakdown voltage.

It is another object of the invention to provide a method for forming acapacitor with a conducting polymer while maintaining a high breakdownvoltage and low ESR.

These and other advantages, as will be realized, are provided in acapacitor. The capacitor has a tantalum anode with an anode wireattached there to. A dielectric film is on the tantalum anode. Aconductive polymer is on the dielectric film. An anode lead is inelectrical contact with the anode wire. A cathode lead is in electricalcontact with the conductive polymer and the capacitor has a break downvoltage of at least 60 V.

Yet another embodiment is provided in a method for forming a capacitorcomprising: compressing tantalum powder into a tantalum anode whereinthe tantalum anode has no more than 0.15 ppm/uC/g oxygen and has no morethan 50 ppm carbon;

anodizing the tantalum anode to form dielectric;

dipping the anodized anode into a slurry of conductive polymer;

drying the conductive polymer; and

providing a first external termination in electrical contact with thetantalum anode and a second external termination in electrical contactwith the conductive polymer.

BRIEF SUMMARY OF THE FIGURES

FIG. 1 illustrates graphically the break down voltage of capacitorscomprising polymeric cathodes compared to the corresponding break downvoltage for capacitors comprising MnO₂ cathodes.

FIG. 2 graphically illustrates DC Leakage Current vs. Voltage of PolymerTantalum Capacitors with Dielectric Formed at 125 V Compared to TantalumCapacitors with a MnO₂ Cathode and a Wet (Sulfuric Acid) Cathode AlsoFormed at 125 V.

FIG. 3 a graphically illustrates initial leakage current distribution ofTa/Ta₂O₅/MnO₂ devices with standard anode (Control) and anode with lowcarbon and oxygen content (Test).

FIG. 3 b graphically illustrates leakage current distribution after 2000hours at 85° C. and 1.32× V rated of Ta/Ta₂O₅/MnO₂ devices with standardanode and anode with low carbon and oxygen content.

FIG. 4 graphically illustrates DC Leakage Current vs. Voltage of PolymerTantalum Capacitors with In-Situ Polymer and Slurry Polymer Compared toTantalum Capacitors with MnO2 and Wet Cathodes.

FIG. 5 illustrates graphically the break down voltage of capacitorscomprising test anodes and polymeric cathodes (both in-situ and slurry)compared to the corresponding break down voltage for capacitorscomprising MnO₂ cathodes.

FIG. 6 illustrates a capacitor of the present invention.

FIG. 7 illustrates a capacitor of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Provided herein is an improved capacitor and method for making theimproved capacitor. More particularly, provided herein is a capacitorcomprising a conducting polymeric cathode with a break down voltage ofover 100 V and ESR of no more than 500 mohms in the range of operatingtemperatures −55 C-125 C. This was previously considered unavailable tothose of skill in the art. More preferably, the capacitor has a breakdown voltage of over 150 V and even more preferably the capacitor has abreak down voltage of over 200 V.

The invention will be described with reference to the various figuresforming an integral part of the instant specification.

Based on previous results with MnO₂ cathodes, wet cathodes, and in-situpolymer based on the same polymer backbone as the slurry polymer, it wasexpected that applying the best anode processing techniques and growingthe best dielectric film would only result in a small improvement ininitial leakage current characteristics. The complexity of applying aslurry polymer has therefore led those of skill in the art to thesimpler in-situ process. However, to our surprise, a synergisticimprovement in leakage current and BDV was realized with a combinationof polymer slurry and anode processing techniques that result in lowconcentrations of both oxygen and carbon in the tantalum. FIGS. 4 and 5illustrate the results graphically. The leakage current and BDVcharacteristics for the combination of polymer slurry deposition and ananode with low oxygen and low carbon content, the “Slurry Test” exceedthose of the Ta—MnO₂ system and are nearly equal to those of theTa—H₂SO₄ system. The results greatly exceed the expectations of askilled artisan when using conductive polymer cathodes.

In general, wet capacitors rapidly increase ESR at low temperature. Ingeneral, higher ESR relates to small case-size parts.

This has led to the unexpected realization that applying slurrycontaining pre-made intrinsically conducting polymer over a tantalumanode with a low concentration of oxygen and carbon provides a capacitorwhich was previously considered impossible. It is most preferred thatthe polymer have a molecular weight of at least about 500 to no morethan about 10,000,000. Below about 500 the polymer chains are ofinsufficient length to offer high conductivity and to form a coatingwith sufficient physical integrity. Above about 10,000,000 the polymericchain is too large to form an adequate slurry.

Formation of a low oxygen and low carbon tantalum anode, and measurementthereof, is provided in “Critical Oxygen Content In Porous Anodes OfSolid Tantalum Capacitors”, Pozdeev-Freeman et al., Journal of MaterialsScience: Materials In Electronics 9, (1998) 309-311 which isincorporated herein by reference. Tantalum powders with a charge of30,000 CV/g or less are preferably used in preparing the anodes. Theparticle size is preferably defined as having an average radius (r) of1.2 μm to 4 μm. It is preferred that the anode have no more than 0.15ppm/uC/g oxygen and more preferably no more than 0.1 ppm/uC/g oxygen. Itis preferred that the anode have no more than 50 ppm carbon and morepreferably no more than 10 ppm carbon.

The invention will be described with reference to the FIG. 6 forming apart of the present application.

In FIG. 6, a cross-sectional view of a capacitor is shown as representedat 1. The capacitor comprises an anode, 2, comprising tantalum. Adielectric layer, 3, is provided on the surface of the anode, 2. Thedielectric layer is preferably formed as an oxide of tantalum as furtherdescribed herein. Coated on the surface of the dielectric layer, 3, is apolymeric conducting layer, 5, which also has carbon and silvertop-coating. An anode lead, 7, and cathode lead, 8, provide contactpoints for attaching the capacitor to a circuit. The entire element,except for the terminus of the leads, is then preferably encased in anexterior moulding, 6, which is preferably an organic and more preferablya epoxy resin.

An alternative embodiment is illustrated in FIG. 7. In Fig. themoulding, 10, may be a hermetic metal casing with the cathode lead, 11,electrically attached thereto preferably by welding. The anode lead, 12,passes through an insulator, 13, to avoid electrical contact with themetal casing.

The anode is typically prepared by pressing tantalum powder andsintering to form a compact. For convenience in handling, the tantalummetal is typically attached to a carrier thereby allowing large numbersof elements to be processed at the same time.

An anode lead is attached to the anode. In one embodiment the anode leadis inserted into the tantalum powder prior to pressing wherein a portionof the anode wire is encased by pressure. For the present invention itis more preferred that the anode lead be welded to the pressed anode.

It is most desirable that the dielectric of the anode be an oxide oftantalum. The oxide is preferably formed by dipping the valve metal intoan electrolyte solution and applying a positive voltage to the valvemetal thereby forming Ta₂O₅.

The formation electrolytes are not particularly limiting herein.Preferred electrolytes for formation of the oxide on the tantalum metalinclude diluted inorganic acids such as sulphuric acid, nitric acid,phosphoric acids, aqueous solutions of dicarboxylic acids, such asammonium adipate. Other materials may be incorporated into the oxidesuch as phosphates, citrates, etc. to impart thermal stability orchemical or hydration resistance to the oxide layer.

The conductive polymer layer is preferably formed by dipping theanodized valve metal anodes into a slurry of intrinsically conductivepolymer. It is preferred that the anode be dipped into the slurry from 1to 15 times to insure internal impregnation of the porous anodes andformation of an adequate external coating. The anode should remain inthe slurry for a period of about 0.5 minute to 2 minutes to allowcomplete slurry coverage of its surface.

The conductive polymer is preferably selected from polyaniline,polypyrrole and polythiophene or substitutional derivatives thereof.

A particularly preferred conducting polymer is illustrated in Formula I:

R¹ and R² of Formula 1 are chosen to prohibit polymerization at theβ-site of the ring. It is most preferred that only α-site polymerizationbe allowed to proceed. Therefore, it is preferred that R¹ and R² are nothydrogen. More preferably, R¹ and R² are α-directors. Therefore, etherlinkages are preferable over alkyl linkages. It is most preferred thatthe groups are small to avoid steric interferences. For these reasons R¹and R² taken together as —O—(CH₂)₂—O— is most preferred.

In Formula 1, X is S or N and most preferable X is S.

R¹ and R² independently represent linear or branched C₁-C₁₆ alkyl orC₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen orOR³; or R¹ and R², taken together, are linear C₁-C₆ alkylene which isunsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen,C₃-C₈ cycloalkyl, phenyl, benzyl, C₁-C₄ alkylphenyl, C₁-C₄ alkoxyphenyl,halophenyl, C₁-C₄ alkylbenzyl, C₁-C₄ alkoxybenzyl or halobenzyl, 5-, 6-,or 7-membered heterocyclic structure containing two oxygen elements. R³preferably represents hydrogen, linear or branched C₁-C₁₆ alkyl orC₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl.

As typically employed in the art, various dopants can be incorporatedinto the polymer during the polymerization process. Dopants can bederived from various acids or salts, including aromatic sulfonic acids,aromatic polysulfonic acids, organic sulfonic acids with hydroxy group,organic sulfonic acids with carboxylhydroxyl group, alicyclic sulfonicacids and benzoquinone sulfonic acids, benzene disulfonic acid,sulfosalicylic acid, sulfoisophthalic acid, camphorsulfonic acid,benzoquinone sulfonic acid, dodecylbenzenesulfonic acid, toluenesulfonicacid. Other suitable dopants include sulfoquinone,anthracenemonosulfonic acid, substituted naphthalenemonosulfonic acid,substituted benzenesulfonic acid or heterocyclic sulfonic acids asexemplified in U.S. Pat. No. 6,381,121 which is included herein byreference thereto.

Binders and cross-linkers can be also incorporated into the conductivepolymer layer if desired. Suitable materials include poly(vinylacetate), polycarbonate, poly(vinyl butyrate), polyacrylates,polymethacrylates, polystyrene, polyacrylonitrile, poly(vinyl chloride),polybutadiene, polyisoprene, polyethers, polyesters, silicones, andpyrrole/acrylate, vinylacetate/acrylate and ethylene/vinyl acetatecopolymers.

Carbon paste layers and silver paste layers are formed for attachingelectrode leads as known in the art. The device is then sealed in ahousing.

Other adjuvants, coatings, and related elements can be incorporated intoa capacitor, as known in the art, without diverting from the presentinvention. Mentioned, as a non-limiting summary include, protectivelayers, multiple capacitive levels, terminals, leads, etc.

EXAMPLES

A comparison of the ESR for devices made from a combination of lowoxygen and carbon anodes and three different cathode systems using H₂SO₄(wet) and MnO₂ each with slurry polymer (poly) measured at roomtemperature is provided in Table 1. In each case the pellet anddielectric formation where identical. The ESR of the polymer system is ½that of the MnO₂ system and ⅕ that of the wet system. Thus, a very lowESR polymer system with a high voltage rating has been realized whichwas previously considered impossible.

TABLE 1 Cathode D L A/Aw ESR (Ohm) Wet 0.1 0.25 1 1.75 MnO2 0.075 0.1233% 0.75 Poly 0.075 0.12 33% 0.3

In Table 1, D is anode diameter, L is anode length, A/Aw is a ratiobetween anode surface in Solid capacitor (A) and Wet capacitor (Aw) andESR is equivalent series resistance in ohms.

Oxygen content in sintered Ta anodes is measured by LECO Oxygen Analyzerand includes oxygen in natural oxide on Ta surface and bulk oxygen in Taparticles. Bulk oxygen content is controlled by period of crystallinelattice of Ta, which is increasing linearly with increasing oxygencontent in Ta until the solubility limit is achieved. This method wasdescribed in “Critical Oxygen Content In Porous Anodes Of Solid TantalumCapacitors”, Pozdeev-Freeman et al., Journal of Materials Science:Materials In Electronics 9, (1998) 309-311 wherein X-ray diffractionanalysis (XRDA) was employed to measure period of crystalline lattice ofTa. According to this invention, oxygen in sintered Ta anodes is limitedto thin natural surface oxide, while the bulk of Ta is practically freeof oxygen.

Another comparison is provided in Table 2 and illustrated graphically inFIG. 5.

TABLE 2 FV In-situ Slurry MnO₂ 9 6.1 26.5 6 15 13 27 14 22 20.5 28.820.5 37 28.9 38.9 32 59 37 57.5 50 74 41.1 80.2 61 93 46 95 75 135 48120 100 210 48 155 118 280 185 120

In Table 2, FV is formation voltage, In-situ refers to in-situ formationof polymeric cathode, Slurry refers to a polymeric cathode prepared byslurry deposition and MnO2 refers to the anode. In each case the anodewas within the inventive levels.

This invention has been described with particular reference to thepreferred embodiments without limit thereto. One of skill in the artwould realize additional embodiments and alterations without deviatingfrom the scope of the invention which is more particularly set forth inthe claims appended hereto.

1. A method for forming a capacitor comprising: compressing tantalumpowder into a tantalum anode wherein said tantalum anode comprises nomore than 0.15 ppm/uC/g oxygen and no more than 50 ppm carbon; anodizingsaid tantalum anode to form dielectric; dipping said anodized anode intoa slurry of conductive polymer; drying said conductive polymer; andproviding a first external termination in electrical contact with saidtantalum anode and a second external termination in electrical contactwith said conductive polymer.
 2. The method for forming a capacitor ofclaim 1 wherein said tantalum anode comprises no more than 0.1 ppm/uC/goxygen.
 3. The method for forming a capacitor of claim 1 wherein saidtantalum anode comprises no more than 10 ppm carbon.
 4. The method forforming a capacitor of claim 1 wherein said conductive polymer isselected from polyaniline, polypyrrole, polythiophene, and derivativesthereof.
 5. The method for forming a capacitor of claim 4 wherein saidconductive polymer is polyethyldioxythiophene.
 6. The method for forminga capacitor of claim 1 wherein said slurry comprises polymeric particleswith molecular weights of about 500 to about 10,000,000.
 7. The methodfor forming a capacitor of claim 1 wherein said capacitor has abreakdown voltage of at least 60V.
 8. The method for forming a capacitorof claim 7 wherein said capacitor has a breakdown voltage of at least70V.
 9. The method for forming a capacitor of claim 8 wherein saidcapacitor has a breakdown voltage of at least 100V.
 10. The method forforming a capacitor of claim 9 wherein said capacitor has a breakdownvoltage of at least 150V.
 11. The method for forming a capacitor ofclaim 10 wherein said capacitor has a breakdown voltage of at least200V.
 12. The method for forming a capacitor of claim 1 wherein saidcapacitor has a ESR of no more than 150 mohms.
 13. The method forforming a capacitor of claim 1 further comprising providing an anodewire in electrical contact with said anode.
 14. The method for forming acapacitor of claim 13 wherein said anode wire is attached to saidtantalum anode by welding in an inert atmosphere.
 15. The method forforming a capacitor of claim 14 wherein said anode wire is tantalum. 16.The method for forming a capacitor of claim 1 further comprisingapplying a carbon layer between said conductive polymer and said secondexternal termination.
 17. The method for forming a capacitor of claim 1further comprising applying a metallic coating between said carbon layerand said second external termination.
 18. The method for forming acapacitor of claim 17 where the metallic coating comprises silver flakein an organic binder.
 19. The method for forming a capacitor of claim 1further comprising forming said dielectric to at least 100 V.
 20. Themethod for forming a capacitor of claim 1 wherein said tantalum powderhas a charge of no more than 30,000 CV/g.
 21. The method for forming acapacitor of claim 1 further comprising at least partially encapsulatingsaid capacitor in an organic molding.
 22. The method for forming acapacitor of claim 1 further comprising hermetically sealing saidcapacitor in a metallic can.
 23. The method for forming a capacitor ofclaim 22 wherein said second external termination is attached to saidcan.
 24. The method for forming a capacitor of claim 1 wherein saiddielectric is Ta₂O₅.
 25. The method for forming a capacitor of claim 1wherein said anode is sintered prior to said anodizing.